1. Field of Invention
The present invention relates generally to reducing interference in a wireless communication system, and more particularly, to methods and systems for reducing interference by canceling interfering radio signals transmitted from wireless devices in neighboring cells.
2. Description of the Prior Art
As wireless communications become more widely used, the demand for wireless resources, such as the available electromagnetic bandwidth, has become increasingly inadequate. One radio communication technique known as Code Division Multiple Access (xe2x80x9cCDMAxe2x80x9d) has been introduced to alleviate some of these problems by allowing simultaneous transmission of multiple signals over a shared portion of the electromagnetic spectrum. This is accomplished by using a spread spectrum technique to assign a unique code to data and modulating the result onto a radio frequency carrier signal. A receiver then detects the radio signal and recovers the data by demodulating the radio signal using that unique spreading code. As a result of the spread spectrum technique, the receiver is able to generate a gain in the power of the detected signal with respect to noise and other signals. Accordingly, signals from a number of transmitters may be contemporaneously transmitted using different spreading codes over the same bandwidth.
Theoretically, if the unique spreading codes were perfectly orthogonal, bandwidth capacity would only be limited by the processing power available at the receiver and the length of the codes. In reality, however, it is unfeasible to design these unique codes to be perfectly orthogonal given the random time offset between signals. Accordingly, interference between contemporaneous signals called multiple access interference (xe2x80x9cMAIxe2x80x9d) results and impacts the capacity and performance of conventional CDMA systems. Although the MAI caused by a single signal is usually small, the amount of MAI becomes substantial as the number of signals increase.
In addition, the amount of MAI can be further exacerbated by high data rate communications. In some CDMA systems, a high data rate communication can be divided into multiple data channels, which prevents too low a spreading factor. In such a situation, each data channel uses a different code and consequently each high data rate communication uses multiple codes. Accordingly, high data rate communications can create artificial hot spots, i.e., one high data rate communication is equivalent to several lower data rate communications all bundled together. For this reason, the MAI impact of a high data rate communication can be more significant than the MAI impact of a lower data rate communication. Similarly, signals originating near the edge of a coverage cell can also exacerbate MAI due to the increased power needed to transmit those signals from the edge of the coverage cell.
Several methods have been used in conventional CDMA detectors to mitigate the effect of MAI. For example, power control has been used to mitigate the near-far effect problem such that all signals, irrespective of distance, arrive at the base station with the minimum power required. The minimum power required for high data rate signals, however, may still be sufficiently great to drown out or severely interfere with other signals. Moreover, limiting the transmit power will generally result in an unwanted consequence of smaller cell sizes.
Another attempt to reduce MAI in wireless communication systems involves subtracting interfering signals from the received signal. Two well-known subtractive interference cancellation detectors are the successive interference cancellation (xe2x80x9cSICxe2x80x9d) detector and parallel interference cancellation (xe2x80x9cPICxe2x80x9d) detector. The SIC detector takes a serial approach to canceling interference. Each stage of this detector makes a bit decision, regenerates, and cancels out one additional signal from the received signal, so that the remaining signals detect less MAI in the next stage. In contrast to the SIC detector, the PIC detector estimates and subtracts out all of the MAI for each signal in parallel. Further information on SIC and PIC detectors may be found in Multi-User Detection for DS-CDMA Communications, S. Moshavi, IEEE Communications Magazine, October 1996, Vol 34, No. 10., herein incorporated by reference.
Prior art cancellation techniques, however, suffer from at least two disadvantages. First, only the signals that are known to the receiving cell are capable of being subtracted. This is because the receiver needs the desired signal""s unique code to demodulate and subtract it from the total received signal. In conventional CDMA systems, those signals are limited to signals from wireless devices within the cell and signals from wireless devices undergoing soft-handoff. Accordingly, interfering signals that are not in soft-handoff or originating within the serving cell are neglected.
In addition, prior art cancellation techniques fail to determine which signal(s) should be subtracted to improve signal reconstruction of the remaining signal(s). Making such a determination is valuable since the processing power required to demodulate and subtract interfering signals is usually limited. The prior art also fails to subtract strategic signals, such as high data rate signals, to significantly reduce MAI.
Accordingly, methods and systems are provided to overcome the above and other problems of the prior art.
Methods and systems consistent with the present invention, as embodied and broadly described herein, reduce interference in a wireless communication system. In one embodiment of the present invention, a first wireless device, being served by a first cell, performs measurements and transmits the measurements to the first cell. Upon receiving the measurements, the first cell determines whether the first wireless device may be causing interference at a second cell and then transmits interference information to a second cell if it is determined that the first wireless device is causing interference at the second cell. Upon receiving the interference information from the first cell, the second cell demodulates and cancels the first wireless device based on the interference information.
In another embodiment, interference between wireless devices is canceled within a plurality of cells serving the wireless devices. The signal strengths of the cells are monitored by the wireless devices and information from the wireless devices is collected at the serving cells. Based on that information, it is determined that one or more of the wireless devices causes interference in one or more of the cells neighboring the serving cells that serve the one or more interfering wireless device. The information about the interference is then transmitted to the neighboring cells to cancel the interference, if the neighboring cells so choose.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description serve to explain the principles of the invention.